The present invention relates to a braking system for braking a rotor relative to a stator, the braking system comprising first braking means secured to the rotor, second braking means secured to the stator, a brake piston suitable for taking up a brake release position and a braking position in which it urges the first and second braking means into braking co-operation, the system further comprising resilient return means continuously urging the brake piston into its braking position, a braking chamber suitable for being fed with a fluid under pressure to urge the brake piston towards its braking position, a brake release chamber suitable for being fed with fluid under pressure to urge the brake piston towards its brake release position, and control means for controlling feeding said chambers with fluid.
For example, the braking system can be the braking system of a hydraulic motor, it being possible for the rotor to be constituted by the cylinder block, or by the casing of the motor, depending on whether the motor is of the rotary cylinder block type or of the rotary cam type. For example, the motor can be a motor having radial pistons, or a motor of the type described in Documents EP 0 869 279 or EP 0 887 548.
The braking means are, for example, constituted by brake plates, constrained to rotate respectively with the stator and with the rotor, and interleaved with one another. Other braking means, e.g. using the teeth of a positive clutch, may be considered.
When the brake release chamber is not fed with fluid under pressure, the brake piston is brought into its braking position by resilient return means. For example, this braking situation applies during parking for a prolonged period.
Under certain conditions, feeding the braking chamber with fluid can be used by way of braking assistance, in order to reinforce the braking effect obtained by the resilient return means.
That is what is proposed, for example, in Documents U.S. Pat. No. 3,680,666, U.S. Pat. No. 4,057,297, and U.S. Pat. No. 3,547,234.
Document U.S. Pat. No. 4,557,109 shows a system in which a chamber containing the resilient return means communicates with the brake release chamber via a restriction which is provided in the brake piston in a manner such that, once the brake piston has reached its brake release position, the restriction is closed. That makes it possible to empty the brake release chamber without having to use a drain or a specific shuttle valve.
As indicated above, during a prolonged halt, the parking braking is used, under the effect of the resilient return means only. In order to allow the rotor to rotate, brake release is performed by feeding the brake release chamber with fluid under pressure so as to displace the brake piston against the return force of the resilient return means. However, it is then sometimes difficult to perform emergency braking. After a prolonged halt, the temperature of the fluid decreases and its viscosity increases. This phenomenon is accentuated when the ambient temperature is low, e.g. lower than 0xc2x0 C. In order to perform braking, it is necessary to cease to feed the brake release chamber with fluid and to empty the fluid therefrom. Because of its high viscosity, the fluid finds it difficult to flow out from the brake release chamber, and so pressure opposing the thrust of the return means remains in said chamber. That prevents the brake from engaging rapidly.
An object of the invention is to remedy that drawback, by making fast and reliable braking possible, even shortly after the rotor has started rotating following a prolonged halt.
This object is achieved by the facts that the braking chamber and the brake release chamber are interconnected via a link duct serving to generate a flow of fluid in an assisted braking situation in which the braking chamber is connected to a fluid feed duct, while the brake release chamber is connected to a fluid return duct, and that the link duct is equipped with means for limiting the flow of the fluid through said duct at least in the direction going from the brake release chamber to the braking chamber.
By means of these provisions, it is possible to brake the rotor rapidly relative to the stator by assisting the return effect of the resilient return means by feeding the braking chamber with fluid, thereby making braking possible even when the fluid is xe2x80x9ccoldxe2x80x9d, i.e. when its viscosity is higher than the viscosity it presents in a stabilized operating situation. At the same time, the braking chamber is connected to the brake release chamber, thereby making it possible for the fluid to flow between the two chambers. This flow contributes to heating the fluid rapidly and thus to decreasing its viscosity, so that the above-mentioned problems disappear rapidly.
The link duct is organized such that, during the assisted braking stage, the forces exerted on the piston due to the resilient return means and to the pressure of fluid in the braking chamber are greater than any antagonistic forces due to the persistence of fluid pressure in the brake release chamber. During the assisted braking stage, the viscous fluid finds it difficult to flow out of the brake release chamber, but it also finds it relatively difficult to flow from braking chamber to the brake release chamber, so that use is made of the high viscosity of the fluid to achieve the assisted braking effect.
However, the flow of the fluid through the link duct is limited in the direction going from the brake release chamber to the braking chamber, so that a conventional brake release situation is nevertheless obtained.
Advantageously, the means for limiting the flow of fluid comprise a non-return valve enabling the fluid to flow only in the direction going from the braking chamber to the brake release chamber.
Advantageously, the system is provided with a restriction disposed in the link duct between the braking chamber and the brake release chamber.
This restriction limits the flow of fluid as a function of its section. It is advantageously disposed in series with the non-return valve in the link duct. In which case, the valve prevents the fluid from flowing through the link duct in the direction going from the brake-release chamber to the braking chamber. In the other direction, the flow is limited by the section of the restriction, thereby making it possible to ensure that a high enough fluid pressure prevails in the braking chamber when the braking chamber is fed with fluid under pressure.
In an advantageous embodiment, the control means for controlling feeding the braking chamber and the brake release chamber with fluid are suitable for causing the system to take up an assisted braking configuration in which the braking chamber communicates with the fluid feed duct while the brake release chamber communicates with the fluid return duct, a brake release configuration in which the brake release chamber communicates with the fluid feed duct, and an unassisted braking configuration in which the brake release chamber communicates with a fluid return, while the braking chamber is isolated from the fluid feed duct.
By means of these provisions, it is possible for the assisted braking configuration to be used only in a critical situation, in particular the situation that occurs on starting, when the viscosity of the fluid is higher than normal. When the critical nature of the situation disappears, the braking can be performed unassisted, without any fluid being consumed to feed the braking chamber.
In which case, the system is advantageously provided with means for detecting a parameter related to the viscosity of the fluid in the braking chamber, and with means for causing braking to take place in the assisted braking configuration whenever the detected parameter reveals viscosity that is greater than a given viscosity, and for causing braking to take place in the unassisted braking configuration whenever said parameter reveals a viscosity lower than said given viscosity.
Thus, when braking is applied, the choice between assisted braking and unassisted braking takes place automatically, as a function of the detected parameter.
For example, this parameter may be chosen from the temperature of the fluid, a head loss, and the temperature of a part of the system that is in contact with the fluid.
Advantageously, the braking means are situated in the brake release chamber. The fluid that is in contact with them is thus heated rapidly.